Using recombinant DNA technology, we have constructed a small library of clones containing inserts which correspond to families of middle repetitive DNA in the mouse. Using the cloned inserts as probes we will determine whether the repetitive families they represent are tandemly clustered or interspersed with other sequences and whether they are located at a single chromosomal site or scattered throughout the genome. Also we will determine whether the different families are transcribed to produce cytoplasmic as well as nuclear RNA and whether the transcripts are a homogenous size class of molecules or heterogenous. Such analyses may provide clues to the functional significance of the families of sequences. By this approach, for example, we may identify candidates for mammalian transposable elements. We also propose to construct larger libraries of middle repetitive sequences from several mouse tissues and study these to determine if any of the families have become repetitive as a result of an amplification event during somatic development. Finally, we intend to study the repetitive elements that might be associated with structural genes coding for several different developmentally regulated mRNAs characteristic for the mouse liver.